We have observed a case of post-synthetic protein modification which results in a decreased specific activity and an increased susceptibility to the cell's degradative system. The protein is bacterial luciferase, a well characterized enzyme which catalyzes the mixed function oxidation of FMNH2 and a long-chain saturated aldehyde by molecular oxygen to yield FMN, the carboxylic acid, and blue-green light. The reaction mechanism appears to be formally analogous to the reactions catalyzed by the flavoprotein monooxygenases or hydroxylases. Because of the ease with which the enzyme can be assayed and because of the availability of a great diversity of mutant forms of the enzyme, it is an ideal model system for study of the monooxygenase-type reactions. Our recent observation of a post-synthetic modification of the luciferase suggests to us that this is also an ideal model system for the study of the basic factors influencing protein "aging" and turnover. The advantages of the system include the quantity of enzyme obtainable, ease of purification, sensitivity of in vivo assay, and the availability of mutants. In fully induced cells, 5 percent of the soluble protein is luciferase, which can be purified to homogeneity in two steps. The enzyme may be easily assayed in vivo without disturbing the cell, merely by measuring light emission, which will allow easy screening for mutants in the cell's protein degradation system. We propose to determine the cause of the modification, the chemical basis for the decreased specific activity, and the role of the cell's degradative system in removing the altered enzyme.